The present invention relates to the control of a digital projector's light source and cooling device.
Digital projectors, hereinafter referred to as projectors, such as portable video projectors used to display computer-generated presentations are frequently employed in business meetings and other venues. The bright images displayed by these projectors and their ready portability are key aspects of their utility. To achieve bright images, these projectors employ special high-intensity, high-temperature light sources. Due to their high operating temperatures, these light sources require forced cooling from a cooling device such as a fan internal to the projector. A typical example of such a light source is the high-pressure mercury vapor lamp, which has an internal pressure of 200 atmospheres and an operating temperature of 1000 degrees Celsius.
These light sources are also subject to operating constraints beyond those of the ordinary incandescent lamp. Specifically, when a mercury vapor lamp is turned off after it has reached operating temperature, it cannot be immediately turned back on or “struck” which refers to the striking of a plasma arc within the lamp. The arc inside the lamp causes liquid mercury to vaporize and give off intense light. Before the lamp can be restruck, it should be cooled below approximately 450 degrees Celsius, i.e. the boiling point of mercury, to where the vaporous mercury has condensed back to its liquid state. Liquid mercury is necessary for striking the arc because vaporous mercury is not an efficient conductor of electricity.
To contend with this operating constraint, projector manufactures force the cooling fan to run for a fixed time period after the projector has been turned off. This is done to cool the light source as rapidly as possible in case the operator might want to turn the projector back on.
However, in practice, once the projector is turned off, it normally remains off until it is needed again in another meeting. Indeed, turning off the projector may signal the end of a meeting, where then the participants may then have to hurry to subsequent appointments. For example, the person making the presentation may have to rush to catch a flight or maintain some other schedule.
The fact that the fan continues to run, even though the projector has been turned off often engenders confusion in the operator. People intuitively expect projectors to act as ordinary appliances in that when they are turned off, they do not continue to run. Moreover, a business person in a hurry to leave for a subsequent appointment may become frustrated not knowing if it is safe to unplug the projector's power cord while the fan is running. Unfortunately, most people caught in this situation feel compelled to wait while the projector's cooling device continues to run.
Like reference numbers and designations in the various drawings indicate like elements.